The present invention relates to light emitting diodes (LEDs) and in particular relates packaged LEDs that are used in side-view surface mount applications and that produce white light.
The basic physics of light emitting diodes is well understood in the art and explained in sources that include, but are not limited to Sze, Physics of Semiconductor Devices, 2d Edition (1981) and Sze, Modern Semiconductor Device Physics (1998). The practical applications of light emitting diodes are also well understood and are explained in helpful terms in a number of sources including LED Lighting Systems, NLPIP Lighting Answers, Volume 7, Issue 3, May 2003, and Schubert, Light Emitting Diodes (Cambridge University Press, 2003).
Side-view surface mount light emitting diodes (also referred to as “side-lookers” or “sidelookers”) are LEDs that are packaged in a manner that transmits their radiation beam parallel to the plane of a circuit board or similar mount. In turn, side looker diodes that can produce white light are useful for incorporation into relatively small devices such as the color screen displays of cellular phones, personal digital assistants (“PDA's”), portable gaming devices, and similar applications.
Such applications often use liquid crystal displays (“LCDs”), polarizing materials, and color filters to create full-color effects. Because typical liquid crystals do not produce light, they are most often used in conjunction with a lighting source and other display elements to produce the desired visible output. For a number of reasons (low cost, long lifetime, reliability), light emitting diodes are frequently used as the light source in such displays. In turn, LEDs that produce white light are particularly useful for such purposes.
In physically small or low power display applications such as cell phones, one design places the white LEDs diodes along the edge or perimeter of the other display elements. When the LEDs are placed in this position, they provide output that is substantially parallel to the display rather than perpendicular to it. Accordingly, diodes that are packaged in a manner that directs their output laterally with respect to a defined plane (usually a circuit board or a display element), are referred to as side-view surface mount diodes or “sidelookers.”
In general, light emitting diodes produce white light using two different approaches. In one approach, multiple LEDs of complimentary hues (e.g., red, green, and blue) are combined to produce white light. In another approach, a light emitting diode that emits in a higher energy portion of the visible spectrum (i.e., blue, violet, or ultraviolet) is used in conjunction with a phosphor that emits in a lower energy region of the visible spectrum; e.g. yellow when excited by the higher energy photons. When properly selected, the combination of the radiation emitted by the diode, and the yellow radiation emitted by the phosphor, produce white light.
The red-green-blue diode approach can offer the advantage of truer color in some circumstances, but typically requires active feedback and control of each LED hue. Alternatively, the single diode with phosphor approach is somewhat simpler in terms of physical construction and circuitry because it requires only the single (usually blue) LED and one or more phosphors, typically carried by an encapsulant adjacent to the diode chip.
For a number of these display applications, visibility represents a primary goal. Thus, obtaining as much light output as possible from any given source remains a fixed goal. In side looker LEDs, however, end users, and sometimes intermediate manufacturers, can experience less visible output then the capability of the underlying diode would indicate. In this regard, the output of the semiconductor diode itself (which will be referred to herein as the “chip”) is often expressed in terms of power; e.g. milliwatts (m W). Because the end use of the diode, however, is a display, its output when packaged and in use is typically measured and expressed in terms of luminous intensity. Luminous intensity is measured in terms of candela (lumens per steradian). Because the candela is defined as the magnitude of an electromagnetic field from a monochromatic light source at 555 nanometers (nm) in a specified direction that has the intensity of 1.46 milliwatts per steradian, the theoretically perfect luminous output of a diode can be calculated from its power output.
In actual practice, however, a number of factors (some of which are unavoidable) reduce the efficiency from the theoretical to a less significant actual output. As one factor, the p-n junctions that produce light in most LEDs have no inherent directional output. Instead, the photons are emitted in all directions from the junction. Thus, some will be absorbed or internally reflected as they move in these different directions.
Other factors that can reduce the output include the amount and composition of the phosphor, its placement, the composition and geometry of encapsulant, and the geometry of the package.
Accordingly, producing brighter displays requires an increase in the output efficiency of side-view surface mount white light emitting diodes.